Process of electrochemically producing amines



Aug. 13, 1968 PROCESS OF ELECTROCHEMICALLY PRODUCING AMINES H. BODE ETAL Filed May 19, 1964 Saturated Colomel Electrode U/Ges KoLomel Elekcrode [mV] Methanol 500- Methonoh Hydruzine 0 ,z 90 {y 1 1000-; Amine Formation Amlnblldung 550 160 1'50 260 mA/cm Saturated Colomel Electrode U/GesKulomel Elekcrode [mv] -Athnnol. 500- AchonohHydruzine Hydrozin 6U0 m Fig. 2

a Amine Formation c. Aminbildung 5'0 160 1%0 260 mA/cm INVENTORS HANS BODE anal MARGARETE JUNG' AGENT United States Patent 2 ,101 15 Claims. (Cl. 204-72) ABSTRACT OF THE DISCLOSURE Amines are produced by an electrochemical process wherein an organic compound which has a hydrogen atom which can be oxidized anodically, for example an alcohol, is dehydrogenated in an electrolyte in the presence of hydrazine in contact with a 'dehydrogenating electrode as the anode at a working potential more positive than the rest potential of the alcohol.

The present invention relates to an improved process of producing chemical compounds electrochemically and more particularly to a process of electrochemically producing chemical compounds by means of radical intermediates.

It is known that numerous reactions, particularly in organic chemistry proceed via intermediates for which the structure of free radicals is assumed. Furthermore, methods are known to demonstrate the presence of such free radical intermediates by intercepting the free radicals by means of suitable compounds capable of reacting therewith. Numerous reactions can be explained only by postulating the presence of free radicals as intermediate stages.

When using electrochemical methods for the preparation of organic compounds, many reactions can be explained by assuming the intermediary presence of free radicals. For instance, saturated hydrocarbons can be produced according to the method first described by Kolbe by electrolysis of saturated solutions of salts of fatty acids. According to Clusiuss theory the fatty acid anions travel to the anode where they give up their charge and form transient free radicals with an unpaired electron on oxygen. These free radicals at once split off carbon dioxide and form free hydrocarbon radicals with an odd electron. Two of said free hydrocarbon radicals combine to form a stable, saturated hydrocarbon molecule of twice the radical size, or the free hydrocarbon radicals may stabilize themselves by disproportioning. Disproportioning of the resulting free radicals is assumed to be the reason for the formation of unsautrated hydrocarbons as by-products. In addition thereto more or less degradation of the starting organic compounds to carbon dioxide and water takes place, even when trying to inhibit the oxidizing effect. Such oxidation is accompanied often by resin formation. Especially when reacting aromatic compounds electrochemically, a number of similar compounds which can be separated with dilficulty only are obtained.

Methods have been described to eliminate the abovementioned difficulties during anodic oxidation. According to these methods, inorganic oxygen transfer agents are added to the electrolyte as depolarizing agents to apply a predetermined potential to the anode. Ions of metals occurring in several stages of valency which are capable of producing redox potentials at the electrode have been found especially suitable for this purpose.

It is one object of the present invention to provide a 3,397,128 Patented Aug. 13, 1968 simple and effective process of electrochemically preparing chemical compounds with the formation of free radical intermediates by which process the difiiculties encountered heretofore are overcome.

Other objects of the present invention and advantageous features thereof will become apparent as the description proceeds.

In principle the electrochemical process according to I the present invention overcomes the difiiculties encountered heretofore by producing the free radicals by dehydrogenation at a constant potential which is as positive as or slightly more positive than the potential required for the formation of the free radicals. These radicals are then allowed to react with chemical compounds or other free radicals.

It is an advantage of the process according to the present invention that compounds acting as depolarizing agents need not be added to the reaction solution. According to the present invention, the free radicals are obtained by dehydrogenation of organic or inorganic compounds at a constant potential which is specific to the respective starting compound used. Keeping the potential constant prevents to a large extent further electrochemical reaction of the reaction products. It is understood that the potential to be maintained is also dependent on the kind of free radicals, further reaction of which to form chemical compounds is desired after addition of other free radicals or other chemical compounds.

According to the present invention, the potential is to be as positive as or slightly more positive than the potential required for the formation of the free radicals. In any case, it must be less negative than the potential which would cause a proceeding dehydrogenation of the chemical compound.

Said potential can be controlled, for instance, by a calomel reference electrode according to known methods and is adjusted by known control devices.

The process according to the present invention can be carried out in an especially advantageous manner by simultaneously producing by dehydrogenation the free radicals to be converted into the desired chemical compounds. Thereby, the potential is appropriately adjusted in such a manner that it corresponds to the desired potential of the more positive free radical.

The compounds to be reacted with each other by means of free radical intermediates are preferably supplied either alone or in mixture with each other to the cathode by addition to the electrolyte. Thereby it is of advantage to separate and divide the electrolyte space between anode and cathode by providing a membrane therein which prevents dilfusion of the reacting and of the resulting compounds to the anode.

The counter electrode may be a cathode on which oxygen is reduced as well as a cathode on which hydrogen is evolved. If the rest potential of the compounds to be dehydrogenated is between the potential of a hydrogen electrode and an oxygen electrode, the procedure depends on the counter electrode. If the working anode is connected to an oxygen-consuming cathode, it is, in principle, possible to produce electric current, i.e. to operate the unit as an electrochemical cell. In contrast thereto, current has to be supplied when dehydrogenating by means of a counter electrode on which hydrogen ions are discharged, i.e. when effecting electrolysis. The process according to the present invention can be carried out in both ways.

Good results are obtained when using the process according to this invention in the production of primary amines from hydrazines and alcohols. The alcohols, for instance, methanol, ethanol, glycol, glycerol, or the like are conducted to the cathode in mixture with a strongly alkaline electrolyte while the hydrazine is separately added drop by drop near the cathode.

By keeping the potential at a value between the rest potential of the electrode with a pure alcohol and base mixture and a value which is by about 100 mv. more positive than said rest potential, amine formation takes place. At voltages below the rest potential, no amine formation takes place but only electrochemical reaction of the hydrazine since its rest potential is more negative. At a much more positive potential the reaction products are further decomposed if they do not escape in gaseous form as, for instance, methylamine and the like.

The attached drawings illustrate voltage diagrams which serve to demonstrate the voltage conditions to be observed when proceeding according to the present invention. In these drawings:

FIG. 1 shows voltage curves of methanol alone and of a mixture of methanol and hydrazine, while FIG. 2 shows voltage curves of ethanol, hydrazine, and a mixture of ethanol and hydrazine.

These curves are plotted in relation to the current load per sq. cm. FIG. 2 clearly shows that the voltage curve of hydrazine is more negative than the voltage curve of the hydrazine-alcohol mixture. The voltage curves of the pure alcohol are shown in FIGS. 1 and 2 for lower loads and are clearly more positive than the voltage curves of the alcohol-hydrazine mixtures. When drawing a horizontal line from the rest potential of the pure alcohol, its intersection with the voltage curve of the alcohol-hydrazine mixture indicates the load value at which amine formation sets in.

All known dehydrogenating electrodes can be employed as working electrodes for carrying out the process of the present invention. Sinter electrodes, preferably those used in fuel cells, yield especially good results. Best results, however, are obtained by using so-called catalyst sieve electrodes as, for instance, described in U.S. Patent No. 3,121,031.

The present process is generally applicable to the electrochemical preparation of compounds provided the rest potentials do not ditfer too widely.

The following examples serve to illustrate the present invention without, however, in any way limiting the same thereto:

In the following examples a carbonyl nickel catalyst activated by means of boronate is used. The catalyst is produced by treating crushed, compressed and sintered bodies of commercial carbonyl nickel with a highly alkaline sodium boron hydride solution. The pore volume of these catalysts is about 80%. The comminuted and activated catalyst is loosely arranged between micro sieves which simultaneously serve as current conductors.

All the examples are carried out at room temperature. The starting materials are dissolved in the electrolyte.

Example 1 750 cc. of 6 N potassium hydroxide solution are placed as electrolyte in the cell and 100 cc. of methanol are dissolved therein. cc. of hydrazine monohydrate are added drop by drop into the immediate vicinity of the dehydrogenating electrode. The rest potential of this cell is 1188 mv. as determined against a saturated calomel electrode. On exposing the cell to a load of 25 ma./sq. cm. and at a potential of 1068 mv., amine formation sets in. On increasing the load to 50 ma./sq. cm. and at a potential of 1020 mv. amine formation is also considerably increased. The amount of methylamine hydrochloride recovered is 6.1 g.

The volatile amine is absorbed during the experiment in concentrated hydrochloric acid diluted with an equal volume of water. Subsequently it is removed from the electrolyte by steam distillation according to known methods. Concentrated hydrochloric acid diluted with an equal volume of water is also used for absorbing the amine from the distilling vapors. The combined hydrochloride solutions are neutralized to yield the free base according to known methods. The resulting base is found to be a primary amine by the isonitrile reaction (Hofmann-isonitrile synthesis). The reactions for secondary and tertiary amines are negative.

Example 2 In place of methanol as used in Example 1 cc. of ethanol are dissolved in 750 cc. of 4.5 N potassium hydroxide solution. The procedure is otherwise the same as described in Example 1. A rest potential of 1188 mv. is determined against a saturated calomel electrode. Amine formation sets in at a potential of 1081 mv. and could be increased under higher load at a potential of 1040 mv. The resulting amine hydrochloride is recovered and isolated as described in Example 1. 4.2 g. of ethylarnine hydrochloride are recovered.

Example 3 On replacing the monovalent alcohols of Example 1 and by an equal amount, by volume, of ethylene glycol and proceeding under otherwise the same conditions, a rest potential of -1180 mv. is determined against a saturated calomel electrode. Amine formation sets in to a noticeable degree at a potential of 1055 mv. and is considerably increased on exposure to a higher load corresponding to a potential of l000 mv. The resulting amine hydrochloride is recovered from the electrolyte by steam distillation.

Example 4 On replacing glycol used in Example 3 by an equal volume of glycerol which is dissolved in the electrolyte, a reset potential of 1160 mv. is determined against a saturated calomel electrode. Amine formation sets in on exposure to a load at a potential of l042 mv. On continuous exposure to a load under a varying potential between 970 mv. and 800 mv., a mixture of amines is obtained in the form of their hydrochlorides and is recovered in the manner described hereinabove.

In place of the alcohols used in the preceding examples, any other alcohol which can be dehydrogenated electrochemically, i.e. which can be oxidized at the anode can be employed for the purpose of this invention. Alcohols with primary and secondary hydroxyl groups are especially suitable. Also any organic compound which has a hydrogen atom that can be oxidized anodically, may be subjected to the process according to this invention.

Of course, when continuing addition of hydrazine, larger amounts of amines than those given in the examples are obtained from the alcohols used.

If it is not contemplated to carry out the process with additional current supply or, respectively, if the price of the second reactant, in the examples of hydrazine monohydrate, is too high to render the production of the amine therefrom economically and commercially feasible, the conventional half-cell arrangement may be employed. Such a half-cell is provided with the dehydrogenation electrode as described in the preceding examples while the counter-electrode is a conventional metal sheet electrode or a carbon electrode, i.e. in general an electrode showing as small as possible an overvoltage. Such counter-electrodes with a low overvoltage are, for instance, platinum wire net electrodes, Raney-nickel double-skeleton catalyst electrodes, or electrodes of arc lamp carbon impregnated with 1 mg. of platinum per sq. cm. of surface. The use of so-called valve electrodes is especially advantageous. Such electrodes are constructed in such a manner that a highly porous covering layer of poorly conductive material is turned towards the electrolyte while a more coarsely porous, catalytically active layer follows and is turned towards the gas space. Due to capillary action the electrolyte decomposes at the boundary between finely porous and coarsely porous layer and the generated hydrogen passes through the coarsely porous layer into the gas space from where it is withdrawn for other use.

The preferred dehydrogenation electrodes need not be too active so as to avoid any cataytic decomposition of the hydrazine. The electrode of carbonyl nickel powder activated by sodium boronate solution has proved of particular value. Other electrodes which may be used as dehydrogenation electrodes are, for instance, the followmg:

Raney nickel double skeleton electrodes, at least part of the active centers of which have been coated by amalgam formation as this is described, for instance, in copending application Ser. No. 325,567 of one of the present inventors Margarete Jung and Hans H. von Doehren, filed Nov. 22, 1963 and entitled Electrode for Electrochemical Devices and Method of its Manufacture.

Nickel sheet electrodes the surface of which has b en alloyed before use with an aluminum layer by a rolling or the like process where-after the aluminum is again dissolved so that the electrode surface consists of a very finely porous layer of nickel. Such electrodes are described, for instance, in German Patent No. 592,130.

So-called economic electrodes as described in British patent spec. No. 909,459 consisting of an electrically conductive support in the form of a plate, a net, or a perforated sheet which has been coated with a thin double skeleton catalyst layer, for instance, of 0.3 mm. thickness and consisting of 60%, by weight, of carbonyl iron and 40%, by weight, of Raney cob alt.

Especially useful for the purpose of the present invention is a carbon cylinder made from a mixture of 80%, by weight, of activated charcoal sold under the trademark Hydratfin and 20%, by weight, of nickel by pressing and heating. When filling such a cylinder with an inner diameter of about 3 cm. to about 5 cm. with a mixture of alcohol and electrolyte, controlled amine formation is achieved by adding the hydrazine hydrate solution drop by drop thereto. Thereby, care is to be taken that the hydrazine contacts the alcohol-electrolyte mixture only near the bottom. This is achieved by introducing the hydra zine hydrate solution through a glass pipette, the opening of which is placed near the bottom. Due to decomposition of the hydrazine into gaseous compounds on contact with the carbon which cannot be completely avoided, excellent mixing of the liquid cylinder content is achieved.

In place of such a carbon cylinder, the carbon electrodes suggested by Kordesch and Marko in US. Patents No. 2,615,932 and 2,669,598 may also be used.

Adjustment of the potential of the dehydrogenation electrode may be effected by any of the known control devices such as they are described, for instance, in copending application Ser. No. 242,296 of the inventors, filed Dec. 4, 1962 and now abandoned, entitled Method of Operating a Fuel Cell and in copending application Ser. No. 284,558 of one of the inventors Margarete Jung and Gerhard Grueneberg, filed May 31, 1963, now Patent No. 3,316,161 and entitled Electrochemical Process of and Apparatus for Replacing Hydrogen in Oxidizable Chemical Compounds by a Functional Group.

As stated above the potential of the dehydrogenation electrode must be adjusted according to the present invention in all instances so that it is at least equal to the potential required for producing the free radical. As shown in the examples and in FIGS. 1 and 2, it may be lower. In general, the range within which the potential is to be adjusted is about 300 mv.

All highly catalytically active cathodes can be used as counter electrodes. Oxygen consuming electrodes are, for instance, silver double skeleton catalyst electrodes or, respectively, cathodes as they are described in copending applications Ser. No. 174,495, now Patent No. 3,231,429 and Ser. No. 174,496, now Patent No. 3,253,961 of one of the inventors Margarete Jung and Hans H. Kroeger, both filed February 20, 1962 and entitled Method for Activating Silver-Containing Electrodes and Method of Activating Silver-Containing Electrodes.

Although the use of alkaline electrolytes has proved to be preferred, it is also possible to effect dehydrogenation in an acid electrolyte as will become evident from the following example:

Example 5 The procedure is the same as described in Example 1 whereby, however, N sulfuric acid is used as electrolyte, in place of 6 N potassium hydroxide solution. The rest potential of the cell is -302 mv. as determined against a saturated calomel electrode. Amine formation sets in at a potential of --179 mv.

Operation in an acid electrolyte, however, has the disadvantage that the resulting amine remains dissolved as acid addition salt in the acid electrolyte and can be isolated therefrom only by rendering the electrolyte solution alkaline. However, when producing other organic compounds which are not soluble in the acid electrolyte by proceeding according to the present invention, an acid electrolyte may well be used.

Of course, many changes and variations in the compounds to be dehydrogenated to form free radicals and in the compounds to be reacted with such free radicals, in the electrolyte, electrodes, and counter electrodes, in the potential at which dehydrogenation is effected, in the manner in which electrochemical reaction with or without current supply is achieved, in the methods of recovering and isolating the reaction products, and the like may be made by those skilled in the art in accordance with the principles set forth herein and in the claims annexed hereto.

We claim:

1. In a process for the electrochemical preparation of an amine by means of a free radical intermediate formed in an electrolyte in contact with a dehydrogenating electrode, the steps which comprise dehydrogenating an alcohol in the presence of hydrazine at a working potential more positive than the rest potential of the alcohol.

2. The process according to claim 1 in which the alcohol is one with a primary or a secondary hydroxyl group.

3. The process according to claim 1 in which the alcohol is methanol, ethanol, glycol or glycerol.

4. The process of claim 1 wherein the potential is between the rest potential of the alcohol and a potential at least about mv. above the rest potential of the alcohol.

5. The process according to claim 1 wherein the electrolyte is an alkaline medium.

6. The process according to claim 1 wherein the electrolyte is an aqueous solution of an alkali metal hydroxide.

7. The process according to claim 1 wherein the amine is a primary alkylamine.

8. The process according to claim 1 wherein the dehydrogenating step is carried out in a half-cell arrangement.

9. The process according to claim 1 wherein the dehydrogenating electrode is made of carbonyl nickel powder activated by sodium boronate solution.

10. The process according to claim 1, wherein dehydrogenation is carried out in an electrochemical cell wherein the counter electrode is an oxygen consuming electrode.

11. The process according to claim 1, wherein dehydrogenation is effected by electrolysis at the dehydrogenating electrode with simultaneous production of hydrogen at the counter electrode.

12. An electrochemical process for the preparation of methylamine wherein a mixture containing hydrazine and methanol is dehydrogenated at a potential between the rest potential of methanol and a potential at least about 100 mv. above the rest potential of methanol in an electrolyte in contact with a dehydrogenating electrode.

13. The process according to claim 12 wherein the potential is from 1068 mv. to 968 mv. and the dehydrogenation is effected in an aqueous solution of an alkali metal hydroxide.

14. An electrochemical process for the preparation of ethylamine wherein a mixture containing hydrazine and ethanol is dehydrogenated at a potential between the rest potential of ethanol and a potential at least about 100 mv. above the rest potential of ethanol in an electrolyte in contact with a dehydrogenating electrode.

15. The process according to claim 14 wherein the potential is from 1081 mv. to -981 mv. and the dehydrogenation is elfected in an aqueous solution of an alkali metal hydroxide.

References Cited UNITED STATES PATENTS 10 JOHN H. MACK, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner. 

